Transient Enhancement and Decoupling of Carbon and Opal Export in Cyclonic Eddies

Abstract

Given their ephemeral nature, eddies have proven difficult to study, with contrary results from field observations that typically sample at the center during a specific stage of an eddy lifespan. Using the natural occurring radionuclide 234Th, we examined particle fluxes within two cyclonic eddies (CEs) at different evolutionary stages (mature stage eddy, C2, and a decay stage eddy, C1) in the oligotrophic western South China Sea (SCS). Using a 1‐D steady state model, 234Th derived particulate organic carbon (POC) and opal fluxes at 100 m were determined. Both POC and opal fluxes were significantly higher at the eddy edges relative to the eddy cores, with integrated eddy POC and opal fluxes of 6.2 (2.1) mmol C m−2 day−1 and 1.5 (0.28) mmol Si m−2 day−1 in C2 (C1). When compared to non‐eddy regimes, both POC (by 2.6‐fold) and opal fluxes (by 7.5‐fold) in C2 were enhanced; they were reduced and decoupled (0.9‐fold and 1.4‐fold) in C1. The difference in enhancement of particle fluxes and the coupling/decoupling between POC and opal flux likely reflects changes in phytoplankton community structures resulting from eddy evolution. Scaling these results to the entire SCS basin suggests that CEs contribute <4% of the net POC flux but >15% of the opal flux. Therefore, CEs may regulate the biogeochemical cycling of silica to a much greater extent than carbon in the ocean.

Cyclonic eddies modulate temporal and spatial decoupling of particulate carbon, nitrogen, and biogenic silica export in the North Pacific Subtropical Gyre

“…Cyclonic eddy‐mediated biogenic Si, particulate C, and N fluxes across the EC and EE during non‐bloom periods were significantly higher than the non‐eddy reference periods (2 months before or after eddy passage, the monthly average or the long‐term annual average). These results agree with the more focused studies of individual wind‐generated mesoscale eddies that form in the lee of the Hawaiian Islands (e.g., Benitez‐Nelson et al 2007; Maiti et al 2008) and cyclonic eddies in the Western South China Sea (Zhou et al 2020), but differ from studies that found little to no enhancement of particulate C and biogenic Si export in the cyclonic eddies of the South China Sea basin (e.g., Yang et al 2015), or seasonally variable particulate C enhancement in the Canary Current region of the North Atlantic Ocean (Alonso‐González et al 2013). We argue that the underlying differences among all of these studies are linked to the evolutionary stage of a single cyclonic eddy and the low spatial resolution of sampling, which often focused on the EC alone.…”

“…Mesoscale cyclonic eddies enhance biological production, influence food web structure, and hence particle formation and export by facilitating the injection of nutrient rich deep water into well‐lit surface waters (McGillicuddy 2016; Resplandy et al 2019; Zhou et al 2020). Understanding the magnitude of cyclonic eddy effects on ocean biogeochemistry is confounded by the inherent spatio‐temporal variability of the open ocean and the difficulty in not only identifying mesoscale cyclonic eddy features, but also comparing eddy‐mediated changes to other, presumably uninfluenced ocean waters.…”

“…Such spatial mismatches between inventory and flux are likely related to delays between biomass response (growth) after nutrient injection and subsequent sinking particle production (see below, Sweeney et al 2003). In other words, once waters are upwelled into the surface, they are physically advected from the EC, where biomass (particulate C and N) initially accumulates (Zhou et al 2020), and into the EE. For biogenic Si, the better coherence in inventories and fluxes is likely due to more rapid sinking of biogenic Si ballasted material relative to particulate C and N.…”

“…It has been estimated that as much as 50% of new production in the global ocean may be due to mesoscale eddy pumping (McGillicuddy 1998). Field observations, however, suggest that significant differences in the magnitude and composition of eddy‐induced particle export occur depending on the mechanism of formation, sampling relative to eddy age, location of sampling within a mesoscale feature, and appropriate non‐eddy references (Benitez‐Nelson et al 2007; McGillicuddy 2016; Zhou et al 2020). For example, while some eddies are characterized by significant particulate carbon (C), particulate nitrogen (N) and biogenic silica (Si) export, others show only enhanced biogenic Si export, or little to no enhancement of elemental particle fluxes (Bidigare et al 2003; Benitez‐Nelson et al 2007; Zhou et al 2020).…”

“…Field observations, however, suggest that significant differences in the magnitude and composition of eddy‐induced particle export occur depending on the mechanism of formation, sampling relative to eddy age, location of sampling within a mesoscale feature, and appropriate non‐eddy references (Benitez‐Nelson et al 2007; McGillicuddy 2016; Zhou et al 2020). For example, while some eddies are characterized by significant particulate carbon (C), particulate nitrogen (N) and biogenic silica (Si) export, others show only enhanced biogenic Si export, or little to no enhancement of elemental particle fluxes (Bidigare et al 2003; Benitez‐Nelson et al 2007; Zhou et al 2020). Therefore, a comprehensive understanding of mesoscale eddy type, the eddy's physical and biogeochemical evolution, and inherent spatial heterogeneity is needed, particularly to understand the larger scale impacts on C and Si biogeochemical cycling in an evolving ocean (IPCC 2019).…”